The present invention relates to methods for enhancing the conductivity of propped fractures in low closure stress subterranean formations.
Non-traditional sources of hydrocarbons are playing an increasingly important role in the oil and gas industry. Such non-traditional sources include gas hydrates, heavy oil, bitumen, and coal bed methane (CBM). Coal bed methane in particular is becoming an increasingly important energy source, with the total coal bed methane reserves in the United States estimated to be between 400 and 850 trillion cubic feet.
Typically, CBM can be found unexploited at relatively shallow depths, and because methane is stored in coal by a different means than conventional gas, more gas per unit volume can be recovered at these shallow depths. In order to extract hydrocarbons from the coal seam, the casing and cement sheet are perforated to create production intervals through which hydrocarbons can flow into the wellbore and ultimately to the surface. To enhance hydrocarbon production, the production intervals are often stimulated by a variety of methods that have been developed and used successfully for increasing the production of CBM from coal seams.
Stimulation operations may involve hydraulic fracturing, acidizing, fracture acidizing, or combinations thereof. Hydraulic fracturing generally includes injecting or pumping a viscous fracturing fluid into a portion of the subterranean formation at a rate and pressure such that fractures are formed or enhanced into the portion of the subterranean formation. The fracture pressure causes the formation to crack which allows the fracturing fluid to enter and extend the crack further into the formation. The fractures tend to propagate is vertical and/or horizontal cracks located radially outward from the well bore.
In such fracturing treatments once the fracturing pressure is released the fractures tend to close, thereby removing at least a portion of the conductivity gained through the fracturing operation. To at least partially prevent closure proppant is generally disposed into the fractures before the fracturing pressure is released. In this way conductive channels remain between the proppant particles in the face of the fracture hydrocarbons to produce from the formation into the well bore.
However, these traditional fracture and propping operations are not specifically suited for use in formations, such as shale and coal bed methane formations wherein the formation itself exhibits a layered structure and fracture “closure” can refer not only to the closure of fracture faces together, but to the closure of individual layers of the formation structure itself.